1. Field of the Invention
This invention pertains to high electron mobility transistors (HEMTs) that employ advanced material designs.
2. Description of Related Art
Future generations of microwave and millimeter-wave radar, communications, electronic warfare, smart weapons, and imaging systems will require higher precision, smaller size, increased bandwidth, lower operating voltages, and lower cost of production. To meet the demand for improved high-frequency performance, considerable effort within the last 10 years or so has focused on the development of gallium arsenide(GaAs)-based and indium phosphide(InP)-based HEMTs. As a result, a variety of HEMT analog and digital circuits have been fabricated which exhibit higher gain, efficiency, and switching speeds with lower power dissipation. The primary factors responsible for the improved HEMT performance have been the increase in the indium mole fraction in the indium-gallium-arsenide (InGaAs) channel and the increase in the conduction band offset at the 2 DEG interface. As a result of these improvements, InP-based HEMTs have distinct millimeter-wave performance advantages compared to GaAs-based HEMTs, and currently hold the record in current gain cutoff frequency response and noise figure for any three-terminal electronic semiconductor device.
In the longer term, HEMTs which use InxAl1-xAsySb1-y, InxAl1-xPySb1-y, or GaxAl1-xAsySb1-y for the barrier layer and InAsySb1-y in the channel may be more attractive than InP-based HEMTs for some of the above applications due to the substantially improved material properties of these new material systems. Higher electron mobility and higher electron velocity may be achieved with a channel composed of InAsySb1-y. The low electron effective mass in InAsySb1-y gives this material a significant advantage in the room-temperature mobility which can be achieved for a given HEMT sheet charge density. InAsySb1-y-based channel materials also have the substantial advantage of a higher electron peak velocity, i.e., 5×107 cm/sec, for pure InSb, compared to the other semiconductors. The large conduction band discontinuity at the donor layer/channel interface enables the formation of a deep quantum well and the associated benefits of a large 2 DEG sheet charge density, good carrier confinement, and high modulation efficiency. These features should enable improved scaling of the current-gain cutoff frequency (fT) as the gate length is reduced to the nanometer range. In addition to the increased high-frequency performance potential, InAsySb1-y channel HEMTs are also attractive for applications requiring low-voltage operation. The higher electron mobility and velocity, lower threshold field and reduced access resistance capability enable the attainment of higher effective velocity at a significantly lower drain voltage.
Although improvements have been made in recent years, the material growth and fabrication technology for antimony-based HEMTs is relatively immature. For the case of AlSb/InAs HEMTs, the high reactivity of AlSb in air and the low valence-band offset of the AlSb/InAs heterojunction increase the complexity of the material growth and device design requirements. For HEMTs with InAsySb1-y channels, barrier layers composed of InxAl1-xAsySb1-y, InxAl1-xPySb1-y, or GaxAl1-xAsySb1-y are required. HEMTs with InAsySb1-y channels, however, are susceptible to significant charge control problems associated with impact ionization in the channel due to the narrow bandgaps of these materials. These effects become increasingly pronounced as the gate length is reduced due to the higher fields present, thus hindering the high-frequency performance of short-gate length HEMTs and limiting their operating voltage range. The combination of Sb-based materials in the channel offers unique opportunities to reduce impact ionization effects by using composite channel layer designs. What is disclosed herein are high electron mobility transistors that employ advanced material designs to increase operating speed and reduce power dissipation.